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Old 06-04-2010, 11:13 AM   #1
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Abnormal AC Cycling


I have a new Lennox AC (13ACD-030) and thermostat (51M32). The AC seems to be cycling abnormally. I have had the installer out to check the system out, but they can't find anything wrong. (They say the refrigerant is fine and the AC is producing cool air - 18 degree differential between vent and return air.) If anyone has any suggestions, I would welcome them.

The system is doing two odd things. The first occurs when the AC is trying to cool my home by several degrees, say from a temperature of 80 to a setpoint of 78. The AC will run for a while (maybe 20 minutes) but then shutoff prior to reaching the setpoint. After a 5 minute delay, it will come on again and run for while but then shutoff again. This continues until the system reaches the setpoint (but not one degree under the setpoint). Is this normal, shutting off after a short run period even though the setpoint hasn't been met?

The second odd behavior occurs when the AC is trying to maintain temperature. For example, if the thermostat temperature is 77 and the setpoint is 77. The AC is turning on before the thermostat temperature rises 78 degrees. And, once it comes on, it turns off before the thermostat temperature lowers to 76 degrees. This cycle occurs continually with the AC running for 5-10 minutes then turning of for 5-10 minutes. My thermostat uses a one degree differential, but the AC is not cooling to one degree below setpoint, and it isn't waiting until one degree above setpoint to turn back on. The installer did try a different thermostat, but it didn't change the behavior.

I would appreciate and welcome any suggestions.




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Old 06-04-2010, 06:47 PM   #2
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Make sure you have proper #18 gauge wiring to the tstat and all the way back to the furnace. NO telephone or small crappy wire! It adds extra load to the circuit and can cause problems. Make sure the hole where the wiring enters the tstat thru in the wall is plugged with pink insulation so no drafts get thru.


"Cut it twice and it is still too short".
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Old 06-04-2010, 08:21 PM   #3
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Thank you. I will check on the wiring. The hole in the wall behind the thermostat is sealed with caulk.

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